As a conventional well-known wheel drive motor for driving a wheel, for example, disclosed are in-wheel motor drive assemblies in Japanese Unexamined Patent Application Publication Nos. 2012-148725 (PTL 1) and 2009-063043 (PTL 2). FIG. 5 shows an in-wheel motor drive assembly 101 as disclosed in PTL 1 and PTL 2, that is mainly composed of a motor section 101A including a motor, a speed reducing section 101B including a cycloid reducer, and a wheel hub bearing section 101C rotatably supporting a wheel hub 108. The motor section 101A has a motor casing 102 that rotatably supports opposite end parts 103a, 103c of a motor shaft 103 using rolling bearings 104, 105. A hollow cylindrical rotor 106 is fixedly coupled on an outer circumferential surface at a central part 103b of the motor shaft 103. An outer circumferential surface of the rotor 106 faces an inner circumferential surface of a stator 107 that is secured along an inner circumference of the motor casing 102. The rotational speed of the motor shaft 103 is reduced by the cycloid reducer (speed reducing section 101B) at a high reduction rate over 1/10 to drive the wheel hub 108. Thus, the cycloid reducer is advantageous over general speed reducers, such as a planetary gear speed reducer and a parallel gear speed reducer, for its compactness and high reduction rate.
The motor shaft 103, rotor 106, and bearings 104, 105 are taken out from FIG. 5 to show them in FIG. 6 at an enlarged scale. The minimum length Ls required for the motor shaft 103 is the sum of an axial dimension L1 required to support the end part 103a of the motor shaft 103, an axial dimension L2+Lf of the central part 103b of the motor shaft 103, an axial dimension L3 required to support the end part 103c of the motor shaft 103, a distance Lp between the end part 103a and the central part 103b, and a distance Lq between the central part 103b and the end part 103c (Ls=L3+Lq+L2+Lf+Lp+L1).